The largest aggregation of spawning American horseshoe crabs in the world occurs in Delaware Bay and supports one of the largest concentrations of shorebirds in the western hemisphere where the birds feed on horseshoe crab eggs during migration. Unregulated harvest in the 1990s is associated with the decline of shorebird populations using the bay, but corresponding baseline information on the horseshoe crab egg food supply that supported peak shorebird populations has been lacking. Past and current measurements of horseshoe crab eggs in the bay indicate that abundance in the 1980s was an order of magnitude greater (x̄ = 156,600/m2) than present‐day estimates (2015–2021 x̄ = 10,243/m2). An additional egg prevalence index, which characterizes the timing and magnitude of horseshoe crab egg output, revealed a similar pattern of higher prevalence in the 1980s (0.89, 95% confidence interval 0.81–0.94) compared with the recent 2015–2021 interval (0.52, 95% confidence interval 0.43–0.60). Declines of egg and shorebird abundance occurred shortly after horseshoe crab harvest reached its peak. Red knot aerial survey counts have stabilized at 30% of the baseline population while ruddy turnstone counts are 40% of baseline estimates. Initially these species were evenly distributed between the New Jersey and Delaware coasts but both species have trended toward predominately occupying New Jersey beaches. A lack of recovery of horseshoe crab egg and shorebird abundance suggests that horseshoe crab harvest management has stabilized populations but progress toward population recovery has been limited. Impacts from bycatch, the pharmaceutical industry and other potential population impacts must be better quantified and mitigated, if necessary, to promote the recovery of horseshoe crab populations. Measurements of horseshoe egg prevalence and abundance are essential metrics for tracking the progress toward management goals for improving shorebird habitat quality.
Around the world, tidal flats play a unique ecological role in estuaries and are a primary feeding habitat for shorebirds and other benthic feeding organisms. Development and economic use of tidal flats can exclude species that depend on this habitat and disrupt ecological processes. In this study we examine patterns of abundance and feeding activity of American horseshoe crabs among oyster aquaculture structures on tidal flats that are adjacent to one of the most important horseshoe crab spawning sites in the world. We used custom-designed traps to sample horseshoe crab abundance beneath rack and bag aquaculture structures and adjacent areas without structures. In addition, we developed predictive spatial models representing three hypotheses regarding the movement of horseshoe crabs through aquaculture structures when transiting to and from spawning beaches. We tested the predictive power of each model using data from traps and found the strongest support for an avoidance model, where on average, horseshoe crabs are avoiding arrays of aquaculture structures when moving across inundated tidal flats. The best-supported spatial model also indicates that patterns of structure avoidance by horseshoe crabs can potentially affect abundance on spawning beaches, particularly with larger gear arrays that are closer to shore. We found additional support for aquaculture structure avoidance by examining an independent data set of horseshoe crab feeding pits on the tidal flats. Patterns of feeding pit density mirrored our trapping results, with fewer pits beneath and among aquaculture structures when compared to adjacent control areas. Horseshoe crabs are important constituents of the benthic food web and their displacement by aquaculture may translate to significant disruptions to the ecological function of tidal flats. This impact can be limited through deliberative spatial planning that seeks to balance ecological and economic management objectives.
Many species of shorebirds migrate long distances from their overwintering grounds in the southern hemisphere to breeding grounds in the northern hemisphere. The coastal intertidal zone, consisting of sand and mud flats exposed at low tide and covered at high tide, is heavily used as a migratory stopover or overwintering habitat. Understanding the spatial distribution of sediment types at these stopover sites is a critical step for understanding habitat use by shorebird species. Due to their importance as overwintering and stopover habitat for the imperiled western Atlantic subpopulation of the shorebird, the red knot (Calidris canutus rufa), as well as other migratory shorebirds, the northern coast of Brazil between Pará and Maranhão, and Bahía Lomas in northern Tierra del Fuego, Chile, were selected for further investigation as to the applicability of remotely sensed characterization of the intertidal flat habitats. Examination of the Landsat 8 multispectral reflectance and Sentinel-1 SAR backscatter reveals that sand and mud represent endmembers at opposite ends of a continuous gradient in feature space. While remotely sensed data can be used to discriminate between mud and sand intertidal types, the spectral relationships varied between the two very different geographic locations. The inclusion of both multispectral and radar sensing imagery can lead to important insights about the physical properties of the sediment that would be omitted by using one data source alone. Spectral unmixing techniques in Google Earth Engine were used to map the intertidal zone into general sediment classes spanning the gradient (i.e., mud, sandy mud, muddy sand, and sand). Comparison of the mapped outputs with field reference data suggests that mapping of mud- vs. sand-dominated areas can be accomplished with reasonable accuracy (overall accuracy of 75%).
Migratory shorebirds are currently at significant risk, with populations of multiple species dropping globally, often due to habitat disruption and loss. The coast of northern Brazil, along the states of Pará and Maranhão, encompasses one of the broadest expanses of shorebird overwintering habitat for many shorebird species in the Atlantic Americas Flyway. Delineating how current management protects shorebird habitat is an important first step to identifying gaps in protections that then need to be prioritized. Different frameworks of conservation decision-making, such as focusing on flagship, umbrella, or biodiversity, can be used to prioritize what habitat needs to be protected. Understanding the effects of these different conservation perspectives is key to making informed management decisions. Multiple dates of Landsat 8 imagery at different tidal stages were used to identify intertidal habitats for further analysis. MaxEnt species distribution models were derived for each species using remote sensing and landscape metrics, as well as point-count survey data collected during the winters of 2016 and 2017. Landsat 8 and Sentinel 2 image data were included as direct inputs to the model rather than interpreted and mapped first into different habitat types. The resulting Maxent models successfully delineated between presence and absence locations for five of the eight target shorebird species. The model outputs were combined to map the intertidal habitat covered under several different management scenarios. These scenarios included using red knots as a flagship species, semipalmated sandpipers as an umbrella species, and species richness as a metric of shorebird diversity. Slightly more than 25% of delineated intertidal habitat was found to be included within a designated Marine Extractive Reserve, with three major gaps in current protections identified. The flagship, umbrella, and biodiversity perspectives result in different prioritizations of these distinct locations. While umbrella and diversity approaches are successful at protecting diverse communities, community assemblages of rarer or specialist species, such as the red knots, may fall outside their protective shadows. A hybrid approach that uses both diversity and the extent of rare flagship species produces results that best meet the management goals of identifying gaps in existing conservation to protect the most at-risk species while conserving the diverse assemblages they coexist with.
Protection of shorebirds along coasts where they congregate during migration is an important conservation goal, yet little is known about the reasons that attract people to beaches. Data on perceptions can be used to develop and implement educational and conservation programs, enhancing both visitor experiences and shorebird survival. Urbanization is encroaching on many beaches and estuaries that are necessary stopover sites for migrant birds. This study examines perceptions of visitors at 9 Delaware Bay beaches during the critical northward shorebird migration using interviews (N = 279). We examine the hypothesis that visitors come to Delaware Bay beaches mainly to see the shorebirds (including Red Knots, Calidris canutus rufa) and the crabs whose eggs sustain the shorebirds. Only 60 % of visitors were there primarily to see the shorebirds and crabs, the rest were there for the scenery, tranquility, shing, or other recreation activities, despite access restrictions. Even when asked what makes the beach attractive, from 17 % (people interested in birds) to 62 % (people interested in crabs) listed tranquility and scenery. Older people rated the importance of research and protection of shorebirds (and crabs) higher than younger people. Over 40 % did not want to see any changes to beaches, while 28 % wanted to see improvements, and 24 % wanted to see more restoration and conservation. Nearly 60% said these beaches would be less attractive if there were more people. These perceptions can aid managers, conservationists and communities improve beaches for both migratory shorebirds and human use.
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